Multiplicity of microsomal cytochrome P-450 is being studied by utilizing monoclonal antibodies (MAbs) to 3-methylcholanthrene (3MC) and phenobarbital (PB) induced rat liver cytochrome P-450. These MAbs directly bind both the corresponding microsomes and purified cytochrome P-450. A semiquantitative, direct radioimmunoassay (RIA) has been developed to measure cytochrome P-450 in the microsomes from various tissues in animals that are untreated, or treated with 3MC or PB. The amounts of cytochrome P-450 in different tissues and species are also being examined by competitive RIA. This analysis provides an approach to the study of cytochrome P-450 multiplicity that is complementary to enzymatic and structural studies. A detailed atlas of the cytochromes P-450 present in different tissues, species, and strains will aid in understanding the diversity of the cytochromes P-450 and their role in individual susceptibility to carcinogenesis.